A well understood problem in the field of telecommunications is that of the quality of a received signal being poor due to local fading and/or interference. Fading can occur in environments where the transmitted signal is reflected in a variety of locations and directions and therefore takes multiple paths between the transmitter and receiver. Where the multipath reflections interfere with one another destructively there is a fade. The location of fades varies with the frequency of the carrier wave used in the transmitted signal. Fading can be a serious problem for narrowband communication systems. (In narrowband systems the bandwidth of a message does not significantly exceed the coherence bandwidth of the channel it is transmitted on. This implies that the channel under consideration is sufficiently narrow that its frequency response can be considered flat.) Interference arises when transmitters within range of a communications device transmit in the same frequency band at the same time as the communications device (or another communications device the first communications device is attempting to listen to).
One method of addressing this problem is to implement frequency hopping. In a frequency hopping system a baseband signal is frequency-mixed up to a variety of radio frequency (RF) channels in a sequence that is known to both the transmitter and receiver so that they “hop” between different channel frequencies. The effects of fading and interference are then diluted as only some of the hop frequencies will be affected. Interleaving and forward error correction (FEC) are used to average out these effects and produce a useful wireless connection. However, in low cost and low power systems, or when short packet lengths are used, interleaving, frequency hopping and FEC may require undesirably large quantities of electrical and/or processing power.
Another way of addressing the fading problem is to employ multiple antennas, spatially separated. The receiver determines which antenna is presently receiving the best signal and preferentially receives with that antenna. However, where small size and/or low cost are important design aims, for example in mobile devices and/or long range chirp communication devices, the additional antenna, receiving circuit, and requisite processing power may not be desirable.
Orthogonal Frequency-Division Multiplexing (OFDM) is sometimes used to address frequency-dependent fading. In OFDM several orthogonal sub-carrier tone signals are used to carry data on parallel data streams or channels. However the composite signals used in OFDM, although composed of narrowband signals, are in fact wideband. Such wideband signals require a faster clock (and hence more circuitry and power) in the receiver than narrowband signals.
What is needed is a communications device, system and method for communications that reduces the effects of fading and/or interference without the disadvantages of the prior art discussed above.